Uniaxial Ratcheting Behaviour of SA 508 Steel

Abstract

In piping materials, such as SA508 steel, used for pressurized heavy water reactors of nuclear power plants, ratcheting is manifested as a critical problem since it may alter the fatigue life of the said structure. Ratcheting is a type of strain accumulation which may occur from the cyclic loading conditions emanating from start-up and shut-down of the plant, variation in operating conditions or particularly in seismic events. It occurs due to asymmetric cyclic loading condition in low cycle fatigue domain. The nature of strain accumulation and associated fatigue life of a material may vary based on different loading parameters such as mean stress, stress amplitude, maximum stress, minimum stress, stress ratio and temperature. Studies on ratcheting behaviour for AISI 304 stainless steel, Cr-Mo steel, carbon steel, aluminium and Cu-alloys have been done by numerous researchers in past few years; although enough scope of work still exists to understand the phenomenon more precisely. It has been recognized that piping material like SA508 steel is prone to ratcheting, but no work is still reported in the literature regarding ratcheting behaviour of steel.
To start working with SA508 pressure vessel steel, the material is first characterized for its chemical composition, microstructural features, hardness and mechanical properties. The microstructure consists bainitic structure. Tensile tests are carried out at nominal strain rate of 1 x10-3s-1 on 7 mm dia.
True stress control cyclic loading tests are carried out in room temperature environment, where effects of stress amplitude (σa) and mean stress (σm) on the ratcheting behaviour are examined. In one set of experiments, σm is varied keeping σa constant, while in a second set σm is kept constant and σa is varied. The chosen σm ranges from positive to zero to negative. An end criterion of the tests is doing cycling up to fracture. It is noted that the accumulation of ratcheting strain increases with increasing σm and σa while the other parameter is constant.
The increase in strain accumulation with increasing magnitude of stress amplitude can be explained with increasing width of stable hysteresis loop with increasing energy of deformation.
Keywords: Ratcheting, Stress amplitude, Mean stress, SA508 steel